1. Field of the Invention
The present invention generally relates to fabrication of semiconductor devices, and in particular, to an electron beam exposure apparatus used in the fabrication of semiconductor devices.
2. Description of the Related Art
Electron beam exposure apparatuses are used in the fabrication of very large scale integrated circuits (VLSI) having an extremely large integration density because of the capability of writing semiconductor patterns with submicron- or even nanometric resolution. As the electron beam exposure apparatus in general has a relatively small throughput, efforts have been made to increase the speed of exposure such that the efficiency of the production of the integrated circuit is improved.
FIG. 1 shows a typical prior art electron beam exposure apparatus. Referring to FIG. 1, the electron beam exposure apparatus includes an electron gun 20 formed from a cathode 21, a grid 22 and an anode 23 for producing an electron beam B. The electron beam B thus produced is passed through an electron lens system including a conversion lens 25, demagnification lenses 26 and 27 and an objective lens 28 after a suitable shaping and is focused on a wafer 30 supported on a movable stage 31. Further, the electron beam B is moved over the wafer 30 by a deflector 29 to be described later. The wafer 30 is applied with an electron beam resist and in response to the irradiation of the electron beam, the part of the electron beam resist hit by the electron beam experiences a chemical reaction. Thereby, the wafer 30 is exposed according to a desired semiconductor pattern.
The semiconductor pattern may be written on the wafer 30 by moving the sharply converged electron beam over the wafer 30 or by passing the electron beam B through a stencil mask to shape the electron beam according to a desired semiconductor pattern. For this purpose, the stencil mask is formed with the semiconductor pattern to be transferred on the wafer by the electron beam. Alternatively, the electron beam B may be passed through a blanking aperture array which selectively passes the electron beam B as an electron beam B1 under a control of an external control circuit to form an array of dots on the wafer 30 by the electron beam B1. In FIG. 1, the blanking aperture array is shown schematically by a reference numeral 32. When passing through the blanking aperture array 32, a part of the electron beam B is deflected away as shown by a beam B2 and is thereby prevented from reaching the wafer 30. For this purpose, the blanking aperture array 32 has a number of deflection electrodes arranged in a row and column formation. Each of these electrodes are energized selectively under control of an external circuit. The foregoing deflector 29 may be an electromagnetic deflector using deflection coils and is provided below the lens 27.
FIG. 2 shows the deflector 29. As shown in FIG. 2, the deflector 29 includes a first coil assembly 291 made of a pair of coils 1 and 2 connected in series for creating a magnetic field Bx1 extending from the coil 1 to the coil 2 in a plane generally perpendicular to the electron beam B, and another pair of coils 3 and 4 also connected in series for creating another magnetic field By1 in a direction perpendicular to the magnetic field Bx1 such that the magnetic field By1 extends from the coil 3 to the coil 4 in a plane generally perpendicular to the electron beam B. Similarly, the deflector 29 includes a second coil assembly 292 made of a pair of coils 5 and 6 connected in series for creating a magnetic field Bx2 extending from the coil 5 to the coil 6 in a plane generally perpendicular to the electron beam B, and another pair of coils 7 and 8 also connected in series for creating another magnetic field By2 such that the magnetic field By2 extends from the coil 7 to the coil 8 in a plane generally perpendicular to the electron beam B and in a direction perpendicular to the magnetic field Bx2. It should be noted that the second coil assembly 292 is disposed closer to the wafer 30 with respect to the first coil assembly 291. Further, the deflector 29 includes a third coil assembly 293 made of a pair of coils 9 and 10 for creating a magnetic field Bx3 extending from the coil 9 to the coil 10 in a plane generally perpendicular to the electron beam B, and another pair of coils 11 and 12 for creating another magnetic field By3 in a direction perpendicular to the magnetic field Bx3 such that the magnetic field By3 extends from the coil 11 to the coil 12 in a plane generally perpendicular to the electron beam B. The third coil assembly 293 is disposed further closer to the wafer 30 with respect to the second coil assembly 292.
In response to the magnetic fields Bx1-Bx3, the electron beam B is moved over the wafer 30 in the X-direction while in response to the magnetic fields By1-By3, the electron beam B is moved over the wafer 30 in the Y-direction. Thus, by controlling the magnetic fields Bx1-Bx3 and the magnetic fields By1-By3 independently, the movement of the electron beam B on the wafer 30 is controlled in mutually perpendicular directions.
Each of the magnetic fields created by the coils 1-11 shown in FIG. 2 are substantially in a plane defined by X- and Y-axes of the coordinate system shown in FIG. 2 wherein the coordinate system has its Z-axis chosen so as to coincide substantially with the general direction of the electron beam B exiting from the electron gun 20 and reaching the wafer 30 (FIG. 1).
In operation, the coils 1 and 2 are energized simultaneously and independently from other coils, the coils 3 and 4 are energized simultaneously and independently from other coils, the coils 5 and 6 are energized simultaneously and independently from other coils, the coils 7 and 8 are energized simultaneously and independently from other coils, the coils 9 and 10 are energized simultaneously and independently from other coils, and the coils 11 and 12 are energized simultaneously and independently from other coils. As a result, the electron beam B is deflected in the X-Y plane independently in the X-direction and Y-direction.
FIG. 3 shows a drive circuit of the deflection system 29. In FIG. 3, only the connection including the coils 1 and 2, the coils 5 and 6 and the coils 9 and 10 for deflecting the electron beam in the X-direction is shown. As shown in FIG. 3, the coils 1, 2, 5, 6, 9 and 10 are connected in series and driven by a drive amplifier 41 which produces a drive current of about 2-3 amperes and about 10 volts in response to a semiconductor pattern data supplied to a deflection controller 43 which produces a digital deflection control data. The digital deflection control data is converted to an analog deflection control signal by a digital-to-analog converter 42 and in response thereto, the drive amplifier produces the drive current which is caused to flow through the coils 1, 2, 5, 6, 9 and 10. The same driving circuit is provided also for the coils 3 and 4, 7 and 8, and 11 and 12.
Each of the coils shown in FIG. 2 has a number of turns of about 5-20 and creates the foregoing magnetic field which act to deflect the electron beam B to a desired location on the wafer 30. In order to achieve the desired deflection with a satisfactory precision, the coils 1-11 are disposed with an exact geometrical relationship with each other and the drive current flowing through the coils are optimized.
In the electron beam exposure apparatus, in general, it is necessary to a) deflect the electron beam by a desired angle, b) deflect the deflected electron beam again so that the electron beam hits substantially vertically the substrate 30, and to c) minimize the coma aberration.
With respect to the foregoing point a), it is desirable and essential that a large deflection angle is obtained for an efficient exposure of the semiconductor pattern.
With respect to the foregoing point b), it should be noted that there occurs a lateral deviation in the beam spot on the top surface of the wafer if the electron beam does not hit the wafer vertically. Referring to FIG. 4 showing an example of exposure of the wafer 30 by a deflected electron beam B, it will be understood that there occurs a lateral deviation of the electron beam B of .DELTA.X in response to the variation .DELTA.H which is a change in the level of the wafer surface with respect to the electron beam B when the electron beam B hits the wafer 30 obliquely. As shown therein, there holds a relation .DELTA.X=.DELTA.H.tan.alpha. where .alpha. represents the deviation of incidence angle from the vertical. When the angle .alpha. is zero and the beam hits the wafer 30 vertically, there occurs no deviation of the beam spot even when the level or height of the wafer 30 is changed because the angle .alpha. is zero. In the conventional electron beam exposure apparatus used currently, however, the parameter tan.alpha. usually takes a value of about 1/20. In this case, the variation of level of the wafer surface 30 of about 10 .mu.m, which is quite common, induces a lateral variation of the electron beam B of about 0.5 .mu.m. As it is required in the electron beam lithography that the semiconductor pattern can be written on the wafer 30 with the line width of less than about 0.5 .mu.m preferably less than 0.01 .mu.m, such a displacement of the electron beam on the wafer 30 is certainly not tolerable.
The coma aberration in the foregoing point c) represents a deformation of the beam spot which cannot be compensated, unlike other aberrations, by the correction coils. In other words, this coma aberration has to be minimized by setting the relation between the coils forming the deflector 29 optimum on the basis of simulation.
In order to satisfy the foregoing points a)-c), the conventional electron beam exposure apparatus of FIG. 1 uses three coil assemblies 291-293, and the geometrical relationship as well as the drive current flowing therethrough are optimized to achieve the desired result with respect to the foregoing points a)-c). More specifically, the coils 1-4, 5-8 and 9-12 forming the coil assemblies 291-293 are adjusted with respect to their vertical positions measured along the path of the beam B as well as with respect to the direction of the magnetic field Bx1-Bx3, By1-By3 particularly the plane perpendicular to the path of the beam B.
The geometrical relationship between the coils 1-11 as well as the construction of each of these coils are determined previously to the assembling of the electron beam exposure apparatus by conducting a simulation, and the coils are manufactured and mounted according to the geometrical relationship determined by the simulation as accurately as possible so that an ideal result is achieved with respect to the foregoing points a)-c). For example, the simulation indicates that a coma aberration of about 0.005 .mu.m can be achieved in the ideal case.
In the actual electron beam exposure apparatus, however, such an ideal state is not achieved. In addition to the commonly occuring variation of the level .DELTA.H of the wafer surface of about 10 .mu.m or more, there arises a problem of error in the mechanical setting of the coils forming the electromagnetic deflector 29. Further, there is an inevitable deviation in the number of turns of the coils forming the electromagnetic deflector as the optimized number of turns of the coils is usually given in a form of non-integer number while the actual number of turns has to be an integer.
For the foregoing reasons, one has to adjust the geometrical relationship by a trial and error process while actually writing a test pattern repeatedly on the wafer 30. Such a trial and error process includes a process of breaking the vacuum and changing the position of the coils within the apparatus by a manual adjusting process. Thus, the adjustment of the electron beam exposure apparatus is complex and requires considerable time. Although the optimization of the deflection of the electron beam may be achieved by changing the drive current independently by using independent driving system for each of the coils 1-12, such an approach needs a number of large and bulky amplifiers capable of providing a drive current of several amperes or more. Associated therewith, the power needed to drive the electromagnetic deflector increases and the cost of the driving circuit also increases steeply.
For the foregoing reasons, the conventional electron beam exposure system had the problems that the electron beam cannot be deflected accurately, the vertical incidence of the electron beam to the wafer is not achieved, and the coma aberration in the magnitude of about 0.1 .mu.m remains.